The present invention relates to semi-conductor memory devices and, in particular, to a differential bit line sense amplifier.
A semi-conductor memory has a plurality of output "bit lines", with each line having a voltage or logic state that is indicative of the data being read from the memory device. The output bit lines are either single-ended or differential and are typically precharged high or low (most often high). During a read operation, the precharge on the bit line is removed and the bit line will then either stay high or fall low, depending upon the data on the bit line.
With differential bit lines only one of the two bit lines in a pair of bit lines will fall low relative to the other bit line, which creates a differential voltage having a polarity indicative of the data being read. A differential sense amplifier senses the differential voltage and generates a single-ended output having a logic state indicative of the differential data. A typical differential sense amplifier includes voltage level shifting circuit, which shifts the voltage levels on the bit lines to where the sense amplifier is most sensitive. Alternatively, the precharge voltage levels can be shifted to place the bit lines at the desired levels.
A difficulty with typical differential sense amplifiers occurs when the amplifier is operated at low power supply voltage levels. As the supply voltage drops, the voltage levels of the differential inputs to the sense amplifier also drop. This reduces the differential voltage presented to the sense amplifier. When both of the sense amplifier inputs drop to a certain voltage level, the sense amplifier will cease to function since the amplifier is not capable of detecting differential voltages when both inputs are below a certain level. One of the inputs to the sense amplifier is required to be above a certain level in order for the sense amplifier to function. Improved differential sense amplifiers are desired, which are capable of operating with lower supply voltages without voltage roll-off at the sense amplifier inputs.